The invention relates to a flow control valve connected between the discharge port of an oil pump and a hydraulic device for controlling the flow rate of oil supplied to the device.
In a power steering apparatus, for example, it is necessary to adjust the supply of hydraulic oil in accordance with the running speed of a vehicle. Specifically, at low speeds, it is necessary to supply an increased amount of hydraulic oil in order to keep the required steering force low. On the contrary, at high speeds, it is necessary that the supply of hydraulic oil be reduced to increase the steering force in order to achieve the running stability. To accommodate for this need, a flow control valve is frequently provided between the power steering apparatus and an oil pump. Such flow control valve may comprise, for example, an orifice and a valve bore formed in a flow path which provides a communication between the apparatus and the oil pump. A spool valve is fitted in the valve bore so as to move forwardly and rearwardly in accordance with a pressure differential across the orifice so that the oil discharge increases in accordance with a vehicle speed or the number of revolutions of the pump. As the pressure differential across the orifice become equal to or exceeds a given value, the spool valve moves in a direction to reduce the opening of the orifice and also opens a return path to a tank, thus returning an excess amount of oil to the tank to thereby reduce the supply of oil to the power steering apparatus.
FIG. 1 shows one examplary form of flow control valve of the kind described. The valve includes a housing 1 in which a valve bore 2 is defined and communicates through a suction passage 3 with the discharge port of an oil pump, not shown, and also communicates with a power steering apparatus, not shown, through a discharge path 4. A return path 5 communicating with a tank, not shown, is connected to the bore. In a region opening into the valve bore 2, the suction passage 3 is formed with a fixed orifice 6 and a variable orifice 7, the opening of which can be varied by a spool valve 9 as can be the opening 8 of the return path 5. Specifically, the spool valve 9 is slidably fitted in the bore 2, and is normally urged by a spring 10 in a direction to block the opening 8. In its inoperative position, the spool valve stays at rest with a stop 11 projecting from the end face of the spool valve 9 bearing against the end wall of the bore 2, thus blocking the return path 5 while fully opening the variable orifice 7. However, when a pressure differential across the orifices 6, 7 becomes equal to or exceeds a given value, the spool valve 9 is moved to the left against the resilience of the spring 10, thus beginning to open the return path 5 as shown, and reducing the opening of the variable orifice 7.
FIG. 2 graphically shows a response of the flow control valve in terms of the flow rate of oil supplied to the power steering apparatus. The response of the described valve is represented by a solid line curve A in FIG. 2. Specifically, at a low number of revolutions of the pump, the oil supply increases with an increase in the number of revolutions. At a number of revolutions N.sub.O, the spool valve 9 begins to open the return path 5, whereby an excess amount of oil is returned to the tank, thus maintaining a constant value Q.sub.1 for the oil supplied to the power steering apparatus. When the number of revolutions increases to a value N.sub.1, the spool valve 9 begins to reduce the opening of the variable orifice 7 and hence the amount of oil discharged into the discharge path 4. When the number of revolutions becomes equal to or exceeds another value N.sub.2, the spool valve 9 completely blocks the variable orifice 7, whereby only the hydraulic oil passing through the fixed orifice 6 is discharged into the path 4, thus maintaining the oil supply at a constant value Q.sub.3.
Assuming that the oil supply Q.sub.3 to the power steering apparatus at the number of revolutions N.sub.2 of the pump represents an ideal value, it follows that the oil supply Q.sub.1 at or near the lower number of revolutions N.sub.1 is not sufficient, producing a steering force which is excessively high. On the other hand, if the opening of the orifice is increased to produce a sufficient oil supply of a value Q.sub.2 at the lower number of revolutions N.sub.1, the opening of this orifice cannot be reduced substantially unless the number of revolutions of the pump increases to a greater value. As shown by a solid line curve B, when the vehicle is running at a high speed, the oil supply cannot be reduced to the value Q.sub.3 unless the number of revolutions increases to a greater value N.sub.3, thus resulting in a disadvantage that the steering force is excessively low at high speeds of the vehicle. As a consequence, it has been difficult in the prior art to provide an ideal response, as represented by a broken line curve C shown in FIG. 2, which assures an optimum oil supply at both high and low speeds of the vehicle.